Request an OAuth access token (and, depending on the grant type, a refresh token).

Supported grant types:

• client_credentials — pass your client_id and client_secret to receive a new access token and refresh token.
• refresh_token — pass your client_id and the existing refresh_token to get a new pair of tokens.
• authorization_code — pass the authCode to get an access token for a specific user. Use this when building your own client.